In a conventional electrophotographic image forming device, friction between a surface of an electrophotographic photoreceptor (referred to also as “photoreceptor”, hereinafter) and a cleaning blade is reduced by supplying a lubricant onto a surface of the electrophotographic photoreceptor so as to prevent a toner for electrostatic latent image development (referred to also as “toner”, hereinafter) from escaping and wearing of the surface of the photoreceptor.
Examples of a method for supplying a lubricant onto a surface of a photoreceptor are (1) using a lubricant application system (applicator), (2) adding a lubricant in a surface layer of a photoreceptor, and (3) adding a lubricant in a developer containing a toner so as to supply the lubricant onto the surface of the photoreceptor at the same time of development.
As for the method (1) using a lubricant application system, an applicator to supply a lubricant on a surface of a photoreceptor may be provided. Although the method has a merit that a lubricant can be supplied on the whole surface of a photoreceptor evenly without influence of blackening area rate of an output image, it makes the image forming device become large and complicated because it necessitates a dedicated device and space and further it causes troublesome maintenance work because uneven lubricant application may occur because a part may be degraded and lubricant refilling means becomes necessary.
When using the method (2) that adds a lubricant in a surface layer of a photoreceptor, although it is effective to suppress wearing of a surface of a photoreceptor to a certain extent, surface characteristics of a photoreceptor varies partially such as partial decrease of sensitivity may occur and thus image defect may be induced.
A method to add a lubricant in a toner is proposed as the method (3). The method, although a toner may aggregate under the presence of excess lubricant and under high temperature and humidity and may cause image deficiency of black spots on a final image, is widely adopted to many types of electrophotographic image forming devices because the method can make the device small and a lubricant can be readily supplied by the method.
Conventionally, a fatty acid metal salt is preferably used as a lubricant of the method (3). Because the slipping property of the salt is preferable, stability of a blade cleaning and suppress of uneven wearing (one-sided wearing) have been examined. For example, Patent Literature 1 discloses a technique to form a fine image without image defect by adding particles of fatty acid metal salt having a diameter of 3 to 15 μm as external additives in toner mother particles so as to increase cleaning ability, suppress wearing of surface of a photoreceptor caused by surface scrubbing with a cleaning blade and stabilize charging property of the toner.
However, if a size of the particles of the fatty acid metal salt is large, the particles of the fatty acid metal salt cannot adhere to the toner mother particles and exist free. As a result, the particles of the fatty acid metal salt adhere to non-image portion on the photoreceptor and are not supplied to a toner developing portion (image portion). Thus the particles of the fatty acid metal salt are not supplied on the whole surface of the photoreceptor.
Patent Literatures 2 to 5 disclose a technique that particles of a fatty acid metal salt having a diameter smaller than a diameter of toner mother particles are externally adhered to the toner mother particles and thus the particles of a fatty acid metal salt are supplied to a toner developing portion on a surface of a photoreceptor with the toner particles at a developing stage.
According to the above art, the particles of a fatty acid metal salt are supplied to the toner developing portion. However, the particles of a fatty acid metal salt are not supplied to a non-developing portion and thus a lubricant is supplied unevenly on the surface of a photoreceptor. As a result, the photoreceptor or cleaning blade is unevenly worn (wearing occurs partially) and it causes decreased life of the cleaning blade. The phenomenon becomes a problem particularly under the circumstance of low temperature and low humidity.